How to measure your pupillary distance (PD) at home

Why PD matters more than people expect

Every spectacle lens has an optical centre, the point where light passes through without being bent sideways. The laboratory positions that centre in front of each pupil using your PD. Put it in the wrong place and the lens behaves as if it had a small prism built in. The rule optical labs use (Prentice’s rule) says the induced prism equals the decentration in centimetres multiplied by the lens power in dioptres. A 2 mm error on −5.00 lenses therefore creates about 1 prism dioptre in each eye, enough to cause eyestrain, headaches or a pulling feeling by the end of a working day.

With a −1.00 lens the same 2 mm error produces a fifth of that and most people never notice. That is why the accuracy you need depends on your power. Progressive lenses are the exception: their narrow reading corridor must line up with your eyes regardless of power, so they always need a precise, monocular measurement.

Single PD or dual PD?

A single (binocular) PD is the total distance between the centres of your two pupils when you look into the distance, for example 63 mm. A dual (monocular) PD gives the distance from the centre of your nose bridge to each pupil separately, for example 31.5 / 31.5, or 32 / 30.5 for a face that is not perfectly symmetrical, which is most faces. Order forms accept either; dual values are better for stronger prescriptions and essential for progressives.

Method 1: a ruler and a mirror

1. Stand about 20 cm from a mirror in good light, with your face level and your glasses off.
2. Hold a millimetre ruler flat against your brow, resting just above your eyes.
3. Close your right eye. Line up the ruler’s zero mark with the centre of your left pupil.
4. Without moving the ruler, open your right eye and close your left.
5. Read the millimetre mark at the centre of your right pupil. That number is your distance PD.
6. Repeat three times. Use the figure that comes up most often, and treat a spread of more than 2 mm as a sign to try the next method.

Method 2: a helper and a ruler

1. Sit facing your helper, about 40 cm apart, with your eyes at the same height.
2. Look at a fixed point in the distance, over your helper’s shoulder. This keeps your eyes in the distance position rather than converging on their face.
3. Your helper rests the ruler on the bridge of your nose, sets zero at the centre of your right pupil and reads the millimetre mark at the centre of your left pupil.
4. For a cleaner reading, they can measure from the outer edge of one iris to the inner edge of the other. Because iris widths are nearly equal, that distance equals your PD, and iris edges are easier to see than pupil centres.
5. For a dual PD, they measure from the centre of your nose bridge to each pupil centre in turn.

Method 3: phone and web tools

Online tools photograph your face while you hold a standard plastic card (85.6 mm wide) against your forehead or chin, then scale the pupil distance from the card. The good ones come within a millimetre of an optician’s pupillometer. The poor ones are thrown off by a tilted head, a card held too far forward or a phone held below eye level.

Treat them as a second opinion. If a tool and your ruler measurement agree within 1 mm, you are done. If they disagree by more, measure again or ask a professional. Some opticians publish guided tools of their own; Optic One, for example, has a PD check page you can use before placing an order anywhere.

Near PD for reading glasses

When you read, your eyes turn inwards, so the distance between your pupils shrinks by roughly 3 mm at a normal reading distance of 35 to 40 cm. Single-vision reading glasses are made to the near PD. If you only have the distance figure, subtract 3 mm: a 63 becomes a 60. Progressive lenses are different again. Give the distance PD and let the lens design handle the inset to the reading zone.

Checking your result

Adult PDs range from about 54 to 74 mm, and most fall between 58 and 68. Children start from around 43 mm and grow into the adult range by their late teens, which is one reason children’s glasses are best measured in a shop. If your reading sits outside the usual range for your face size, measure again before you trust it.

Your current glasses are another cross-check. An optician can find the optical centres with a focimeter in a minute and read the PD they were made to. Some online stores offer to work out PD from a photo of you wearing your existing glasses; it is a rougher method, so use it only to confirm a figure you have already measured.